1. Field of the Invention
The present invention relates to optical disc apparatuses and more particularly, to an optical disc apparatus which, when an optical disc is subjected to a random access operation to record/reproduce an information signal to/from a land/groove track on the optical disc, performs counting operation of the number of tracks and also performs discriminating operation between inner and outer peripheries of the optical disc.
2. Description of the Related Art
There has been rapidly studied and developed an external storage which has both of the high-speed accessibility of a magnetic disc for computer use and the large memory capacity of an optical disc. Optical disc apparatuses will be positively destined to lead the next generation.
FIG. 7 shows an exemplary arrangement of a paior art optical disc apparatus which includes a laser light source 51, a collimator lens 52, a composite prism 57, an objective lens 58, a convex lens 60, a beam splitter 61, an RF optical sensor 62, a two-division optical sensor 63, and amplifiers 64 and 65. The composite prism 57 has a wedge part 53, a polarization beam splitter part 54, a 45-degree mirror part 55, and a 1/4 wavelength plate 56.
Explanation will be made as to the operation of the above optical disc apparatus. A light beam emitted from the laser light source 51 is collimated by the collimator lens 52; then passed through the wedge part 53, polarization beam splitter part 54, 45-degree mirror part 55 and 1/4 wavelength plate 56 in this order; and then directed to the objective lens 58 to be focused on an optical disc 59. The light beam reflected by the optical disc 59 goes back to the 1/4 wavelength plate 56, where the reflected light beam is subjected to a circular polarization of rotation of a direction opposite to the incident light beam to the optical disc 59 to be converted to a linear polarization beam perpendicular to the incident beam, reflected by the 45-degree mirror part 55, and then directed into the polarization beam splitter part 54.
The incident light beam is called S wave for the polarization beam splitter part 54, where the incident light beam is deflected by an angle of 90 degrees, focused by the convex lens 60, partly reflected by the beam splitter 61, and then directed to the RF optical sensor 62; whereas, the remaining light beam is passed through the beam splitter 61, and then received by the two-division optical sensor 63 whose division line is parallel to a track tangential direction of the optical disc 59, so that the two-division optical sensor 63 generates a tracking error signal based on a so-called push-pull method. The RF optical sensor 62 detects an RF signal on the basis of 90-degree deflected light wave. Two electric signals delivered from two sensing elements A and B in the two-division optical sensor 63 are amplified by the amplifiers 64 and 65, and then output therefrom as output signals T.sub.A and T.sub.B.
FIGS. 8(A), 8(B), 8(C) and 8(D) show a relation between tracks of the optical disc 59 and a beam for explaining a way for counting tracks in a prior art land/groove optical disc. In detail, FIG. 8(A) shows a cross-sectional view of a part of the optical disc 59, in which L denotes a land and G denotes a groove. FIG. 8(B) shows a configuration of the beam focused on the land (L)/groove (G) with respect to the two sensing elements A and B of the two-element optical sensor 63. That is, the two-element optical sensor 63 is positioned so that the division line of the two-division optical sensor 63 is parallel to the track tangential direction 70 of the optical disc 59.
In FIG. 8(B), a white part 71 denotes the land (L) on the optical disc 59 and a hatched part 72 denotes the groove (G) on the optical disc 59. It is assumed in the drawing that the output signals T.sub.A and T.sub.B from the two sensing elements A and B in the two-division optical sensor 63 are arranged so that, for convenience of explanation, the output levels thereof are made large when the focused beam comes to the land part while are made small when the beam comes to the groove part. Then, in FIG. 8(B), when the focused beam moves transversely across the tracks toward a rightward direction in a random access mode, the output signal T.sub.A from one sensing element A is as shown by 73 in FIG. 8(C), while the output signal T.sub.B from the other sensing element B is shown by 74 in FIG. 8(D).
In the random access mode of the optical disc apparatus, a tracking error signal generated based on the output signals T.sub.A and T.sub.B varies whenever the light beam transverses each track. Thus, the optical disc apparatus, by counting the tracking error signal, can judge the number of tracks to reach on a target track and an optical head therein can move the focused light beam to the target track at a high speed.
According to the prior art optical disc apparatus mentioned above, when a width ratio of the optical disc between the land and groove is set to be 1:1 or to be a value close thereto for high density recording with respect to a disc radial direction and when the tracking error signal is detected by the two-division optical sensor 63, the two output signals T.sub.A and T.sub.B from the two sensing elements become to have a phase difference of about 180 degrees as shown by 73 and 74 in FIG. 8(C) and 8(D) in the random access mode. For the sake of the phase relationship, the prior art optical disc apparatus has a problem that the apparatus cannot judge on the basis of such a resultant tracking error signal whether the focused beam is moving toward the inner peripheral side of the optical disc or toward the outer peripheral side thereof, which results in that, even when the apparatus counts the tracking error signal, the apparatus cannot judge a distance to the target track.